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Heterogeneous photocatalysis involves the use of a semi-conducting material
which can be excited by the absorption of light. The applications of photocatalysis
include water treatment and purification, air treatment and purification, and
'self-cleaning' surfaces. Photosynthetic applications are also widely
reported including photoelectrolytic water splitting, CO2 reduction
and organic synthesis. There are a wide range of materials employed in photocatalytic
research and applications. The important properties of these materials include
the band gap energy and hence the wavelength of light required for excitation,
the chemical and photochemical stability, particle size and surface area.
The use of nano-structured materials may lead to improved photocatalytic efficiencies
where the reduction in particle size results in a greater surface area and possibly
size quantisation effects. The former provides more active sites for reaction
and the latter gives an increase in the absorption coefficient at specific wavelengths.
The most commonly employed photocatalyst material for research and industrial
applications is titanium dioxide (TiO2). This is because it is photostable,
chemically stable, photoactive, relatively inexpensive and non-toxic.
There are various routes to producing nanostructured titania including sol
gel, hydrothermal, electrochemical oxidation of titanium, chemical vapour deposition
and plasma sputtering deposition. Titania has a band gap energy of around 3.2
eV and therefore is a UV absorber. When excited by UV irradiation the electron-hole
pairs in the titania can react with water and dissolved oxygen to form reactive
oxygen species which can attack organic (and inorganic) pollutants in the water.
Effectively, each excited particle becomes a nano-electrochemical cell driving
redox reactions at the interface. Dispersed nano-structured (but most likely
aggregated) titania can be utilised for water treatment and purification.
Suspensions have been used in lab based research and even on large scale treatment
systems, however, the catalyst must be recovered from the water prior to discharge.
Alternatively, the catalyst may be immobilised as thick or thin films on a solid
supporting substrate to negate the catalyst recovery stage1.
For water treatment applications, the use of immobilised films presents problems
for reactor design due to mass transfer limitations2.
If the catalyst is immobilised onto an electrically conducting supporting substrate,
one can employ this substrate as a photoanode in a photoelectrochemical cell
(PEC), either in photolytic or photogalvanic mode). For example, one application
may be the solar driven photocatalytic oxidation of organics and the simultaneous
reduction of dissolved metal ions in a two compartment PEC3.
Photocatalysis has been reported to be effective against a wide range of chemical
pollutants including persistent organic pollutants (POPs). One interesting application
investigated at Ulster
was the destruction of the female hormone 17-β-oestradiol and it's analogues.
Hormones and hormone mimics are termed endocrine disrupting chemicals EDCs and
pose a significant threat to the environment.
Photocatalysis is a degradative process where attack by reactive oxygen species
results in the overall oxidation of an organic pollutant via intermediate products.
These intermediates may be just as harmful as the parent compound. In relation
to the oestrogen compounds, it is important to determine the destruction of
the oestrogenic properties. This was achieved using a yeast screen bioassay
which responds to the oestrogenic effect of pollutants. It was shown that photocatalysis
was more effective than UVA photolysis in destroying the oestrogenic effect
of 17-B-oestradiol, esterone and estriol4,5.
More recent research has demonstrated the photocatalytic destruction of pharmaceuticals
in water. Given that photocatalysis generates reactive oxygen species including
hydroxyl radical, superoxide radical anion and hydrogen peroxide, it is a logical
step to apply the treatment towards the disinfection of water containing pathogenic
microogranisms. Indeed, photocatalysis has been reported to be effective against
a wide range of microoganisms including bacteria, viruses and protozoa.
| E. coli
is the name of a type of bacteria that lives in your intestines. Most
types of E. coli are harmless. However, some types can make you sick
and cause diarrhea. One type causes travelers' diarrhea. The worst type
of E. coli causes bloody diarrhea, and can sometimes cause kidney failure
and even death. These problems are most likely to occur in children
and in adults with weak immune systems.
Clostridium perfringens is an anaerobic,
Gram-positive, sporeforming rod (anaerobic means unable to grow in the
presence of free oxygen). It is widely distributed in the environment
and frequently occurs in the intestines of humans and many domestic
and feral animals.
Cryptosporidium parvum is one of several
species that cause cryptosporidiosis, a parasitic disease of the mammalian
intestinal tract.
|
Our work at Ulster
has investigated the inactivation of E.coli as a model organism6
using photocatalysis and electrochemically assisted photocatalysis. In the latter,
the process is assisted by the application of an external electrical bias. While
E.coli is a pathogen in its own right and is used as an indicator for faecal
contamination, it is relatively easy to kill. Therefore, it is more interesting
to study the inactivation of disinfection resistant species.
We have shown that photocatalysis and electrochemically assisted photocatalysis
are effective against the spores of Clostridium perfringens7.
Furthermore, we have also demonstrated that photocatalysis is effective against
the protozoan oocysts of Cryptosporidium parvum. This organism is a
big problem for the water industry because it is resistant to conventional disinfection
and causes severe diarrhoea in humans.
Ulster is a partner
in the EC FP6 Sodiswater
project which aims to investigate the solar disinfection of water for use
in developing countries. By simply filling a transparent bottle with water (preferably
glass or PET) and placing in direct sunlight, one can inactivate most pathogens
in the water, therefore rendering the water safer to drink. Given that around
one sixth of the World's population do not have access to safe water, it makes
sense to utilise the power of the sun in such a simple process.
While SODIS (solar disinfection) is used throughout the world by around 2
million people, the uptake for SODIS could be improved. Additionally, there
is a need for improvements in SODIS efficiency and quality assurance for the
end user.
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To this end we have been investigating the use of photocatalysis to enhance
the rate of kill of pathogens at pilot scale under real sun conditions at the
Plataforma Solar de Almeria in collaboration with Pilar Fernandez, CIEMAT, Spain.
Also, we have been developing sensor technologies to provide automated control
and quality assurance for the end user. Our collaborators include partners in
Kenya, Zimbabwe and South Africa. Nanotechnology could help save lives in the
developing world.
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Reference
1. Byrne J.A., Eggins B.R., Brown N.M.D., McKinney B., and
Rouse M., Immobilisation of TiO2 powder for the treatment of polluted water.
Applied Catalysis B: Environmental, 1998, 17, pp 25-36.
2. McMurray, T.A., Byrne, J.A., Dunlop,P.S.M., Winkelman, J.G.M.,
Eggins, B.R., and McAdams, E.T., "Intrinsic kinetics of photocatalytic oxidation
of formic and oxalic acid on immobilised TiO2 films." Applied Catalysis A: General,
2004, 262, 1, 105-110.
3. Byrne J.A., Eggins B.R., Byers W., and Brown N.M.D., Photoelectrochemical
cell for the combined photocatalytic oxidation of organic pollutants and the
recovery of metals from waste waters. Applied Catalysis B: Environmental, 1999,
20, L85.
4. Coleman H.M., Eggins B.R., Byrne J.A., Palmer F.L., and King
E., Photocatalytic degradation of 17-ß-oestradiol, Appl. Catal. B Environmental,
2000, 24, L1 - L5.
5. Coleman, H.M., Routledge, E.J., Sumpter, J.P., Eggins, B.R.,
and Byrne, J.A., "Rapid loss of estrogenicity of steroid estrogens by UVA photolysis
and photocatalysis over an immobilised titanium dioxide catalyst," Water Research,
2004, 38, 3233-3240
6. Dunlop, P.S.M., Byrne, J.A., Manga, N., and Eggins, B.R.,
The photocatalytic removal of bacterial pollutants from drinking water, Journal
of Photochemistry and Photobiology A: Chemistry, 2002, 148, pp 355-363.
7. Dunlop P S M, McMurray T A, Hamilton J W J, Byrne J A, "Photocatalytic
inactivation of Clostridium perfringens spores on TiO2 electrodes", Journal
of Photochemistry and Photobiology A: Chemistry, 2008, 196, 113-119
Copyright AZoNano.com, Dr. John Byrne (University of Ulster)